Styrene/butadiene rubber latexes are used in adhesives for carpet backing to prevent the unraveling of the carpet fibers and are also used in the paper industry as coatings. One of the problems associated with the latexes is microbial (particularly bacterial) contamination of the uncured latex. Several problems ensue when formulated latex coatings, adhesives, or backings become contaminated. These include an offensive odor, which remains in the latex after it is cured and renders the resulting carpet or other product undesirable for use, changes in rheology of the adhesive or backing, which prohibit uniform coating of a carpet-back surface and make the viscosity of the adhesive or backing unsuitable for use, and reduced surface properties, which cause an adhesive or backing to lose strength and which can result in delamination of a carpet from its backing.
Contamination is most often noticed first by odor. When the microbial count exceeds 100,000 microbes per ml, odor begins to become apparent. The offending microbes can include bacteria, fungi (and yeasts), algae and other microscopic forms of life. Although yeasts and fungi can present problems, most problems in the spoilage of carpet backing compounds are caused by bacteria. Gram-negative bacteria are of particular importance in this regard as they are generally viewed as causing more degradation than gram-positive bacteria. For example, Pseudomonas aeruginosa, which is widely distributed in nature and is found in the water, soil, and intestinal tract of man and animals and consequently, is a commonly occurring bacterial contaminant. See "Test Method D4783-89" published by the American Society for Testing and Materials for further information concerning common contaminants.
Contamination of latex backing or adhesive can have several sources. The first source may be the finished latex itself. If the latex is not properly treated, it may introduce contaminants into the carpet mill where it is formulated. The scrim adhesive-type latexes used in carpet backcoatings are all carboxylated; the foam backing types (cold polymerized SBR latex) are not. In some cases, contamination results from shipping or extensive storage of finished latex.
The second source of potential problems is the raw materials used with finished latex. Typically, the styrene-butadiene latex backcoatings are heavily filled with calcium carbonate or clay, which are sources of microbial contamination. Unsterilized water has also been used as dilution, washing or carrying agent. Many times recycled water is used for dilution of latex. The industry is moving towards recycling water because of environmental, safety and economic concerns, and the recycled water presents an ideal environment for microbial growth. Commonly added defoamers, surfactants and viscosifiers could also be bacteria carriers.
Another source of potential problems is the storage of compounded latex. Storage of the compounded latex in storage tanks where temperature and time of storage are not strictly controlled all pose opportunities for introduction and growth of bacteria.
In the past, latex suppliers have added microbicide to the latex to control contamination in the carpet mills. Such microbicides include for example, 1,2-benzoisothiazolin-3-one, which is sold by Imperial Chemical Industries under the trade name Proxel GXL, and a mix of 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one, which is sold by Rohm and Haas under the trade name Kathlon LX, and a mix of 1,2-dibromo-2,4-dicyanobutane and 1,2-benzoisothiazolin, which is sold by Calgon under the trade name Biocheck 410. However, these products suffer from certain shortcomings. For example, high concentrations of 1,2-benzoisothiazolin-30-one, are required to adequately preserve the latex.
Tetrahydro-3,5-dimethyl-2H-1,3,5-thiadiazine-2-thione (Nalco 5787) and methylene(bisthiocyanate)(Nalco 5793) have also been used in carpet backing adhesives. Nalco 5787 is a preservative at high dosages but has odor problems and does not offer quick kill capabilities. Nalco 5793 is a good quick kill biocide but does not offer preservation. Therefore, it is apparent that an improved method and composition for inhibiting growth of microorganisms (and bacteria in particular) in latex backing formulations is needed.